Distribution automation plays an important role in the protection of the electricity dis-tribution network from the different type of faults. However there is always space for the improvements in this field. The main aim of the protection of the network from faults is to safe human beings, properties and avoids long service breaks. This in return will reduce the outage duration and outage costs. Nowadays, customers want the con-tinuous supply of power for their business and home without any interruptions. The de-mand for continuous power supply has forced electricity distribution companies to im-prove the quality of the supply. Due to which the maintenance cost is increased. Thus still there is need for the development of techniques which will reduce the fault frequen-cy and enable more efficient protective methods in order to avoid long outage durations and damages done by the faults in the distribution network.
In Finland over 80% of the annual outage costs of customers are due to faults in public medium voltage (MV) distribution networks. Out of these faults most of outage cost is due to the permanent faults. It is estimated that about over 90% faults are tempo-rary which can be cleared by auto-reclosing and below 10 % are permanent. Among permanent faults about 50% are earth faults. Many techniques have been developed in order to detect the earth fault even the high resistance earth faults.
In medium voltage network, the steady state behavior of the protection system along with its dynamic behavior is influenced by the way how the neutral of the distribution system is earthed. Distribution system operators (DSO), working in Finland, have long experience of operating the 20 kV system with the isolated neutral or as compensated system. The resistivity of earth in Finland is very high which can lead to small earth fault currents in isolated systems but there are some type of earth faults where the fault current can even be more than usual earth fault and act as like short circuit faults. These types of earth faults are usually termed as cross country earth faults.
1.1. Objectives and content of thesis
This thesis focuses on the method development to detect the cross country earth faults and to separate these faults from other types of the faults in medium voltage network.
The main idea of the developed method is based on the change in the phase currents and all combinations of sum of two phase currents. The method detects the cross country faults and protects the distribution network from them.
Medium voltage network consisting of three feeders was modeled in simulator. The model was used for the method development and for the testing purpose. The method uses the triggering signal from the directional earth fault protection function. After that faulty feeders and faulty phases are determined by calculating the change in combina-tions of sum of two phase currents and phase currents on each feeder respectively. The
measured combinations of sum of two phase currents are tested for defined limits to separate the cross country faults from the other type of faults. This method is designed to implement in the systems based on the concept of centralized protection and control.
Chapter 2 discusses the theory of the faults in medium voltage network in Finland and their protection methods. Chapter 3 explains the modern central protection system and role of IEC 61850 standard. Moreover this chapter also throws light on the ongoing research project of centralized protection system of ABB and some of its protection functionalities used in medium voltage network and implemented its IEDs. Chapter 4 is written in order to give the idea of the simulation environment before going into details of the developed method. The novel developed method for detecting the cross country earth fault is explained in detail in chapter 5. This chapter includes the description of the flow chart and basics of method along with the explanation of the method with an ex-ample. Chapters 6 and 7 show the results of the simulation environment as described in chapter 4 in different fault scenarios. Chapter 8 discusses the future aspects of the meth-od and its implementation in real medium voltage networks. In the end chapter 9 con-cludes the thesis along with the observation and success of the method.
2. Distribution network and fault types
Distribution network is the back bone in the power transmission of any country. The power is generated by power plants and reached to the customers through the transmis-sion and distribution network. In order to supply reliable and cheap power to the cus-tomers, it is necessary to protect the network from the faults. The faults can be of differ-ent types e.g. short circuit or earth faults etc.
This thesis is dealing with the protection of the network from the cross country earth faults in the medium voltage (MV) network. Cross country faults are type of earth faults in which faulty phases are short circuited faults through the ground. Thatβs why a meth-od is needed to detect these faults and protect the network from the short circuit cur-rents. In cross country earth faults the short circuit between the phases on same or dif-ferent feeders occur through the ground. Before going into details of the cross country faults, it is necessary to have a look on the structure of the distribution network and the parts of the networks where cross country faults can occur. This chapter of thesis is fo-cused on the structure of distribution network in Finland, type of faults in medium volt-age network and existing methods to safe the network from cross country faults, to de-tect them and separate them from the other faults.
2.1. Finnish distribution network characteristics
Electricity distribution system is different in different countries. The structure of the main distribution network in the country depends upon the requirements of the country, sources for generation and geographical territories in that country. For example in Fin-land, loads currents are separated from the neutral and returning currents through the earth due to high ground resistance. In this method power is supplied to the loads be-tween the phases (i.e. positive and negative sequence parameters provide the infor-mation of the power supplied to loads). The zero sequence parameter is used for the earth fault detection. The technique of detection of fault by zero sequence parameters is used in high voltage and medium voltage network. In low voltage (LV) network has four wire systems and the neutral point is earthed. One advantage of earthed four wire system is that MV network is not affected if there is an earth fault in the LV network.
[4] [2]
In Finland three levels of voltages are used in the distribution networks. These volt-age levels are 110 kV, 20 kV and 400 V for the high voltvolt-age, medium voltvolt-age and low voltage networks respectively. [13] The main features of distributing network of Fin-land are as follows [3]:
- Primary substations (main substation or feeding substation) normally provides with one or more 110/20 kV transformers fed by power transmission network - Medium voltage (20 kV, sometimes 10 kV) feeders
- Switching substations along some feeders having only circuit breakers - Distribution substations equipped with a 20/0.4 kV transformer
- A low voltage network with 400 V voltage level
- Network can be isolated neutral or compensated neutral
Voltage level 400 kV is used, as Extensive High Voltage (EHV), for the long dis-tance power transmission in Finland from generation sources to the primary substations.
Figure 2.1 shows the basic structure of the transmission and distribution network in Fin-land. [4]
Figure 2.1 Basic structure of transmission and distribution systems in Finland. [5]
As there is no neutral wire in the MV voltage networks, therefore these networks are divided into isolated neutral network or compensated neutral network categories. These categories are explained in the next section.
2.2. Isolated and compensated networks in Finland
As said in the earlier section, the medium voltage network has the three wire system.
This means that there is no neutral/earth wire. In medium voltage network the primary substation transformer can be in delta configuration or in the star configuration. In delta configuration there is no neutral point so there is no need for the neutral connection to the earth. Sometimes in delta configuration the primary transformer is forced to make a neutral point through an earthing transformer. In the case of star configuration we have the neutral point automatically. The importance of neutral point can be seen in the case of the earth faults. In the power systems, different ways of neutral treatments have been developed for the protection of the system from the over voltages, the need to restrict the touch potentials etc. depending upon the voltage levels. [6] The neutral treatment is classified generally as isolated neutral or the compensated neutral hence networks are called as isolated network and compensated network respectively. In isolated network the neutral point is left as it is while in compensated network the neutral point is earthed via an arc-suppression coil known as the Petersen coil. This coil lowers capacitive earth fault current and also avoid over voltages in network [5].
In Finland nearly 50% of the medium distribution networks are isolated. The com-pensation in the medium voltage network can also be done by the implementation of several compensated coils along the distribution network depending upon the earth fault current (i.e. decentralized compensation). [7] Due to different behaviors of the fault currents in isolated and compensated network, there is need of different methods for the fault detections. In the next section some background of the single phase earth faults has been explained for the isolated and compensated systems.
2.3. Faults types in MV network
2.3.1. Single phase earth fault in isolated network
In the isolated network, the currents of the single phase to ground faults depend mostly on the phase to earth capacitances of the transmission line. In the event of the fault, the capacitance of the faulted phase is by passed as a result system become unsymmetrical.
Then the fault current is composed of the capacitive currents of the healthy phases [6].
The phenomena of single phase to ground fault is shown in figure 2.2.
Figure 2.2 Single phase to ground fault with an isolated neutral. [6]
The impedances of the network except the capacitive earth impedances are very small so they can be neglected. The phase to earth capacitances is denoted as πΆπ. The theveninβs equivalent model of the isolated network in the case of the earth fault is show in figure 2.3
Figure 2.3 Thevenin equivalent circuit in case of single phase to ground fault in the isolated neutral network. [6]
In the case of when π π = 0 , the fault current can be calculated by equation 2.1 [6]:
πΌπ = 3ππΆππΈ (2.1)
Where π = 2ππ is the angular frequency of the network. While in the case when there is some fault resistance, the fault current can be found through equation 2.2. [6]
πΌππ = πΌπ
β1+(πΌππΈπ π)2
(2.2)
Where πΌπ is obtained from above equation 2.1. It is also observed that when the single phase to ground fault occurs the voltage levels in the healthy phases increases. Due to this overvoltage phenomenon the chances of the cross country earth fault increases. The voltages in the healthy phases increases according the vector diagram of the voltages which is shown figure 2.4. [6]
Figure 2.4 Voltage vectors during the single phase to ground fault in isolated neutral network. [6]
2.3.2. Single phase earth fault in compensated network
The compensated systems are also known as the resonant earthing system. In this type of network the capacitance current is compensated by the inductive current provided by the compensated coil. The circuit is parallel resonance circuit and in the case of full compensation only the resistive part of the fault current is left .The resistive current is due to the resistance of the coil and the resistive part of the distribution lines together with the system leakage resistance (π π) . In order to make the selective relay protection to be implemented there is need of specific amount of the fault current. Therefore some-times parallel resistance π πΏ is used to increase the fault current. The compensated net-work looks like in figure 2.5 in case of single phase earth fault as below. [6]
Figure 2.5 Single phase to ground fault with an compensated neutral. [6]
The thevenin equivalent circuit for the phenomena of the single phase to ground fault in the compensated network is shown in figure 2.6. [6]
Figure 2.6 Thevenin equivalent circuit in case of single phase to ground fault in the compensated neutral network. [6]
Using the equivalent Thevenin circuit we can write the fault current equation 2.3. [6]
πΌππ = πΈβ1+π 02 (3ππΆ0β
1 ππΏ)2
β(π π+π 0)2+π π2π 02(3ππΆ0β(ππΏ)21 )2
(2.3)
In case of exact compensation the equation 2.3 can be reduced to πΌππ =π πΈ
π+π π . In com-pensated systems the phase to earth voltages of the two healthy phases behaves similar to isolated system. Compensation reduces the fault current provided by the capacitive discharging
2.3.3. Short circuit and phase to phase to earth faults
The short circuit faults are the most common type of faults. These faults are divided in to the two phase short circuit fault and three phase short circuit fault. In short circuit faults, phases touch each other directly or through some fault resistance due to which the heavy current flows through the breakers and when these inrush currents are higher than the specified limits the breakers are opened and hence save the network from being collapsed.
The behavior of short circuit fault changes when one of the short circuited phases al-so experiences the earth fault. This type of fault is known as the phase to phase to earth fault or double phase earth fault. Usually the reason for this type of fault is that when there is the single phase earth fault the voltage of the healthy phases rises. The rise in the voltages leads to the flashover or break down between the earth fault phase and the one of the healthy phase. Phase to phase to earth fault can be shown in figure 2.7 along with their equivalent symmetrical components model. [6]
Fig 2.7 The phase to phase to earth fault and corresponding connection of symmetrical component sequence networks. [6]
The currents flowing in different phases can be found by the equations below πΌπΏ1 = βπΈπΏ1β πππΆπ (2.4) πΌπΏ2 = βπβ3πΈπΏ1(π π0+3π πβππ2
1π2+(π1+π2)(π0+3π π)) β πΈπΏ1β πππΆπ (2.5) πΌπΏ3 = +πβ3πΈπΏ1(π π0+3π πβππ2
1π2+(π1+π2)(π0+3π π)) β πΈπΏ1β πππΆπ (2.6)
In equation 2.4 πΆπ is capacitance of phase conductor to ground while in equations 2.5 and 2.6 π0, π1 and π2 are zero, positive and negative sequence impedances respective-ly. The line currents are composed of the capacitive current along with load currents because the system is isolated neutral. The figure 2.7 shows the flow of the capacitive currents as case of phase to phase to earth fault. The equations 2.4, 2.5 and 2.6 will be
used to find the limits values which are used in the algorithm developed in the thesis.
The information about the limits and the method to find them is explained in chapter 5.
PHASE C
PHASE B
PHASE A
Capacitive Current of Phase A Capacitive Current of Phase B Capacitive Current of Phase C
Short Circuit current btween phase A & B
Figure 2.8 Flow of capacitive currents along with short circuit current in case of phase to phase to earth fault between the phase A and phase B.
In figure 2.8 the phases A and B are under the phase to phase to earth fault. In this fault the location of the short circuit and phase to earth fault is same. Due to this the capacitive current due to the discharge of phase A and B conductorsβ capacitances is same or different in case of fault resistance while the capacitive current from phase C conductors will distribute in phase A and B conductors according to the resistance of the short circuit between phase A and B and the earth fault resistance. In this way the phase A conductor will has current consisting of capacitive current from phase A, B, C and the short circuit current but the capacitive current of phase B entering to phase A conductor and the phase B capacitive current coming through the source side adds to zero current. Same is case for conductor of phase B. In this way only the capacitive cur-rent of phase C conductor will occur in phase A and B conductors along with short cir-cuit current.
2.3.4. Cross country earth fault
Cross country faults are type of two phases to earth faults. In this type of fault the both the phase experience a phase to ground fault separately and the phases are short circuited through the ground. In Finland, mostly medium voltage networks are installed in radial topology. In the case of a short circuit in cross country fault, short circuit cur-rent may be smaller than the predefined limit of overcurcur-rent protection relay due to ground resistance. Hence they are not easy to detect. While in case of the directional current relays the currents and their angles will exist out of the operation region of relay.
Due to which the faults are not detected. The cross country fault is divided into two cat-egories.
- Cross country fault on the same feeder - Cross country fault on different feeders
In cross country fault on the same feeder, two separate phases are experiencing the phase to ground fault independently and the location of the faults are different along the same feeder. In this way the two phases are short circuited through the ground and there is earth resistance along with fault resistances between two phases which are short cir-cuited. This type of fault is shown in the figure 2.9. [6]
Figure 2.9 Cross country fault on same feeder. [6]
One of the reason for the occurrence of this type of fault is that when the one phase ex-periences the phase to ground fault then due to the phenomena of the over voltages on the healthy phases increases the chances of the other phase to undergone the earth fault.
In cross country earth fault on different feeders, two separate phases on separate feeders have undergone the phase to ground fault. Again the phenomenon of short cir-cuit between the faulty phases occurs through the ground. It must be noted that phases must be different for the cross country fault on different feeders. If the phases are same then they will be detected by the directional earth fault protection relays and hence the network can be protected. The cross country fault on different feeders is shown in figure 2.10. [6]
Figure 2.10 Cross country fault on different feeder. [6]
The common reason for this type of fault is that if the earth fault occurs then the over voltages increase the chance of phase to ground fault in the healthy phases on the other feeders of same primary substation. The figure 2.10 shows the flow of capacitive currents due to the discharge of the capacitances of the conductors of the phases along